Semiconductor lasers capable of producing continuous stimulated radiation at wavelengths in the vicinity of 1.1-1.7 um (micrometers) at room temperature are of interest for communications systems using fiber optics, since it is in this wavelength range that both the transmission losses and the dispersion in high-quality glass fibers are low.
Semiconductor lasers of quaternary III-V alloys of GaInAsP grown on a binary compound of InP (double-heterostructures or DH) have proven particularly satisfactory for operation at this frequency range. Furthermore, the distributed feedback (DFB) type of laser construction has been of particular importance for producing narrow spectral width stable mode lasers.
The genesis of the DFB laser goes back to Kogelnik et al., "Coupled-Wave Theory of Distributed Feedback Lasers", J. Appl. Phys., Vol. 43, No. 5, (May 1972) pp. 2,327-2,335). In essence, a DFB laser comprises a light waveguide with a grating or periodic structure adjacent thereto. The spacing of the perturbations of the periodic structure are selected to be an integral number of half wavelengths of the desired light frequency within the laser, such that the perturbations produce Bragg Scattering which couples and reinforces right and left light waves traveling through the light guiding layer in a coherent manner such that reflections are in phase [See U.S. Pat. No. 3,970,958, issued July 20, 1976]. Deviations in periodicity of the grating structure are therefore highly undesirable.
More recently, DH type DFB laser devices have been constructed [See, for example, K. Sakai, "1.5 um Range InGaAsP/InP Distributed Feedback Lasers", IEEE J. Quantum Electron., QE-18, 1272 (1982)]. These lasers combine the advantages of the DH structures with the frequency stability attainable with DFB gratings.
Recently, a novel technique for burying the active layer where fabricating, non-DFB, BH type lasers was described in U.S. Pat. No. 4,468,850 to Liau et al. issued Sept. 4, 1984, which utilizes a "mass transport phenomenon" to bury the active layer. Briefly, in this mass-transport formed DH laser, an active layer of quaternary III-V alloy is grown on a binary III-V compound substrate and a top layer of a binary III-V compound is similarly epitaxially grown on the active layer. An oxide stripe mask is formed by conventional photolithography techniques on the top layer.
An undercut mesa structure is then formed by a two-step selective chemical etch. A first etchant is used to remove the top layer where it is not protected by the oxide coating. This top layer is removed down to the active quaternary layer at which point the first etchant step is immediately terminated and a new etchant is used to remove the active layer underneath the remaining top layer, except for a thin volume of active material underlying the remaining top mesa structure.
Next, the structure is subjected to a controlled temperature cycle which produces a transport of material so as to fill in the void left at the undercut region and thereby enclose the sides of the remaining volume of the active material. Then, ohmic contacts are provided across the device to enable current to be passed through the structure to produce lasing.
Despite the above improvements in technology which extend over a decade, a need still exists for a DH-DFB laser of relatively simple construction which can be produced with high yield and performance.
A chief cause for the complexity and low yield of the prior devices is (1) the requirement of close optical coupling between the grating and the active layer waveguide region coupled with (2) the necessity of isolating the ohmic contacts of the laser diode from the grating structure to prevent loss of optical power caused by absorption in the metal contacts. Moreover, usually the metallization for the ohmic contact is sintered, or alloyed. If metals are alloyed into the semiconductor grating structure the grating structure melts and its performance deteriorates. Hence the mutually conflicting problem that the metallization must be kept distant from the guiding layer but the grating relatively close.
This has led the prior art workers to locate the grating under the active layer, or to locate the grating over the active layer and then grow an isolating layer on the grating. The ohmic contact is then formed over the isolating layer.
Both of these approaches suffer from the distinct disadvantages that:
(1) mass transport of the grating material can occur while heating the wafer prior to growth of layer(s) over the grating. This smooths out the grating and reduces its height and hence the coupling of the light between grating and waveguide region. It is particularly difficult to achieve a first-order grating in which the tooth spacing equals half the wavelength in such a manner because of the extremely small dimensions. For a laser emitting at a wavelength of 13 um the internal wavelength is about 4000 .ANG..
(2) it is difficult to obtain good uniformity and good morphology in layers grown over a non-planar corrugated grating structure;
(3) the internal wavelength of the laser and, hence, the desired grating periodicity depend upon the exact value of the optical index of the dielectric waveguide formed. Until all the layers are grown and the actual thicknesses of each are known, the optical index of the dielectric waveguide is not precisely known.